The use of natural gas as public utility fuel gas has had the advantage that in distributing pipe systems which are under relatively high pressures (about 30 kilograms per square centimeter absolute pressure), the natural gas containing more than 90% methane may be used to transport the highest possible calorific value per unit of volume. These supply systems are centrally fed from the sources of natural gas. In view of the large areas covered by these systems it is necessary to produce a synthetic natural gas from other fuels, particularly liquid fuels, in order to compensate for pressure fluctuations during times of high gas consumption or to supplement overloaded or exhausting sources of natural gas.
DAS No. 1,180,481 discloses a process in which evaporable liquid hydrocarbons are cracked on high-nickel catalysts at temperatures of about 450.degree.C to produce a rich gas which has a relatively high methane content, contains relatively little carbon monoxide and has a much higher calorific value than coke oven gas, which was previously considered a standard supply gas.
The methane content of this rich gas can be increased by subjecting it to a further reaction on another catalyst at temperatures below 400.degree.C whereby the hydrogen content is reduced by the hydrogenation of CO and CO.sub.2.
DOS No. 1,645,840 discloses a process for methanating a rich gas, wherein the oxides of carbon are hydrogenated in two stages and at least in the first stage are hydrogenated in the presence of the water vapor which has not been reacted during the production of the rich gas. The rich gas leaving the reactor is cooled to a temperature above its dew point and is then catalytically reacted in the first methanation stage. Because of the high water vapor content the heat which is liberated during the methanation reaction results only in a moderate temperature rise of about 50.degree.-60.degree.C. Because water vapor is a product of the methanation reaction, the quantity of steam which is already present prevents a reaction of a considerable quantity of hydrogen with the oxides of carbon, and the temperature rise in the catalyst layer displaces the equilibrium in the undesired direction toward the starting substances. For this reason, in the second methanation stage, the inlet temperature is reduced and by a cooling below the dew point the water vapor content is reduced to such an extent that the carbon black limit defined by the Boudouard reaction is not reached during the further reaction.
It has been believed that the large quantity of water vapor must be carried along at least through the first methanation stage in order to hold down the temperature rise due to the hydrogenating reaction, to prevent a formation of carbon black by the Boudouard reaction.
On the other hand, the presence of water vapor involves considerable disadvantages. The relatively large vapor volume must be moved over the catalyst as a ballast material and tends to shift the equilibrium of the methanation reaction in the undesired direction. For this reason the vapor also reduces the rate of the methanation reactions so that, particularly in the first stage, only small space velocities are possible and correspondingly large quantities of catalyst are required. From among the large number of the so-called high-activity methanation catalysts, only those are suitable which are insensitive to the high concentration of water vapor in the gas to be reacted. Besides, between the two methanation stages the entire gas must be cooled to condense water vapor and must be reheated to the inlet temperature of the second methanation stage.
Plants for producing synthetic natural gas must handle gas at very high rates, and the high water vapor rates require uneconomically large heat exchangers, reactors, and quantities of catalysts.
DOS No. 1,545,463 discloses a process for producing a high-methane gas wherein evaporable hydrocarbons are initially cracked with 1.5-3 kilograms water vapor per kilogram of hydrocarbon on a catalyst which comprises nickel or cobalt on a magnesium silicate support to produce a rich gas having a relatively low water vapor content. The rich gas is subsequently methanated and for this purpose is first cooled to 200.degree.-250.degree.C. and passed over an indirectly cooled methanation catalyst. A single methanation stage is used to give an end product which when scrubbed to remove carbon dioxide contains more than 98% methane by volume. It has been found, however, that the process cannot be carried out on a large scale unless the operating conditions are controlled with very small tolerances.
To produce a gas which consists almost entirely of methane, the hydrogenation of carbon monoxide and carbon dioxide must be controlled so that the hydrogen is consumed as completely as possible. This is accomplished by the following reactions: EQU CO + 3 H.sub.2 .fwdarw.CH.sub.4 + H.sub.2 O EQU co.sub.2 + 4 h.sub.2 .fwdarw.ch.sub.4 +2 h.sub.2 o.
a gas produced by cracking of evaporable hydrocarbons with water vapor on nickel-containing catalysts at temperatures above 450.degree.C and under a pressure of 25 kilograms per square centimeter at a feedstock ratio of 2.5 kilograms water vapor per kilogram of hydrocarbons (boiling range 30.degree.-110.degree. C) has approximately the following composition on a dry basis:
CO.sub.2 23% by volume CO 1% by volume H.sub.2 18% by volume CH.sub.4 58% by volume
The gas also contains 1.14 standard cubic meters of water vapor per standard cubic meter of gas.
This gas is in a thermodynamic equilibrium. To permit hydrogenation of the oxides of carbon until the hydrogen has been substantially consumed, the required thermodynamic conditions must be met by a reduction of the temperature, resulting in a change of the equilibrium constants, and/or by removing at least part of the water vapor. The difficulties which are involved in carrying out this methanation process are due to the extremely exothermic character of the above-mentioned hydrogenating reactions. 50 kilocalories of heat are liberated by the reaction of one mole CO.sub.2 with 4 moles H.sub.2. This means a liberation of 2 kilocalories by the reaction of 1 liter CO.sub.2 (0.degree. C., 760 millimeters mercury). In 1 standard cubic meter of rich gas of the above listed composition, the reaction of 10 liters carbon dioxide and 40 liters hydrogen results in a temperature rise of about 35.degree.C. The temperature rise in the reaction mixture shifts the equilibria of the two hydrogenating reactions to the left, toward the starting materials, and may finally result in a higher heat loading of the catalyst. If the temperature-reducing action of water vapor is utilized by leaving it in the rich gas, as e.g., in the wet methanation described in DOS No. 1,645,840, the above-mentioned disadvantages must be accepted.